This is a lead bulla, a seal attached to papal documents to prove their authenticity. One face gives the name of the Pope, the other reads SPA SPE above the heads of Saint Paul and Saint Peter. The design changed very little for hundreds of years and one bulla looks very like another except for the name of the Pope, which makes them easy to date. Unfortunately, lead is easily damaged and worn, so some are hard to read. This example is quite clear, and was identified by Dr Tim Pestell (Norfolk Museums Service) as a bulla of Boniface IX (1389 - 1404).